Preparation of Chiral Amino Esters by Asymmetric Phase‐Transfer Catalyzed Alkylations of Schiff Bases in a Ball Mill

Abstract

The asymmetric alkylation of Schiff bases under basic conditions in a ball mill was performed. The starting Schiff bases of glycine were prepared beforehand by milling protected glycine hydrochloride and benzophenone imine, in the absence of solvent. The Schiff base was then reacted with a halogenated derivative in a ball mill in the presence of KOH. By adding a chiral ammonium salt derived from cinchonidine, the reaction proceeded asymmetrically under phase-transfer catalysis conditions, giving excellent yields and enantiomeric excesses up to 75 %. Because an equimolar amount of starting material was used, purification was greatly simplified.

CHAPTER 4. Asymmetric Organocatalytic Reactions under Ball Milling

“…Over a period of 1 h under solvent-free reaction conditions, it was possible to obtain the desired compounds with excellent yields (in the range 91-97%) and with up to 75% ee (Scheme 4.10). 45 …”

Metal-catalyzed organic reactions using mechanochemistry

“…The pioneering work by Bolm 77 on asymmetric organocatalyzed aldol reactions, followed by the contributions by Najera, 78 Hernández and Juaristi,79 Šebesta, 80 Xu, 81 and Lamaty, 82 …”

Recoverable Dendritic Phase‐Transfer Catalysts that Contain (+)‐Cinchonine‐Derived Ammonium Salts

“…[26,27,32,33] All electrophiles used were commercially available. Products 9-22 were previously described in the literature, 18a,31 1 H, 13 C NMR and chiral GC retention time are indicated to probe purity.…”